fix(grader): Workaround ocaml-sf#457

Graders using module `Introspection` (which used to work with 0.12)
should now work with `ocamlsf/learn-ocaml:master` as well, i.e., they
should not raise `Ctype.Unification_trace.Unify(_)` any longer.

href: ocaml-sf#457 (comment)

src/grader/grading.ml

@@ -139,6 +139,10 @@ let get_grade
       handle_error (internal_error [%i"while preparing the tests"]) @@
       Toploop_ext.use_string ~print_outcome ~ppf_answer
         "module Report = Learnocaml_report" ;
+      (* The following 3 lines are just a workaround for issue #457 *)
+      handle_error (internal_error [%i"while preparing the tests"]) @@
+      Toploop_ext.use_string ~print_outcome ~ppf_answer
+        "module Introspection = Introspection" ;
       set_progress [%i"Launching the test bench."] ;
 
       let () =